They are not "bleeding red ink" they are just being gouged and forced to fund things even the largest corporations do not do.
Pensions at the post office are probably the most secure retirement fund in the US right now, well other than what congress awards itself.
The pension costs of future workers are simply required to be included in future projections of USPS costs. It used to be that the USPS could go to congress and say "we want to do X over the next 10 years, it will cost $Y". The $Y value was allowed to exclude the cost of retirement benefits for workers hired during those 10 years. Now it must be included.
This is simply good accounting practice.
The government is requiring USPS to set aside money for benefits which it has already promised, which is a no-brainer. Did you know that health benefits for employees in the private sector are not promised, and they have been yanked away?
According to Wikipedia:
During the 1990s and 2000s, many employers who promised lifetime health coverage to their retirees limited or eliminated those benefits. ERISA does not provide for vesting of health care benefits in the way that employees become vested in their accrued pension benefits. Employees and retirees who were promised lifetime health coverage may be able to enforce those promises by suing the employer for breach of contract, or by challenging the right of the health benefit plan to change its plan documents in order to eliminate those promised benefits.